The signaling molecule nitric oxide (NO) is critically involved in airway and vascular function in the developing lung. NO is produced by three isoforms of NO synthase (NOS), neuronal NOS (nNOS), endothelial NOS (eNOS), and inducible NOS (iNOS). Our preliminary work in normal baboon fetuses indicates that all three isoforms are expressed in airway epithelium, and that nNOS and eNOS are upregulated during late gestation to optimize NO production in the perinatal period. Bronchopulmonary dysplasia (BPD) is an inflammatory condition which disrupts the development of the preterm human lung, and it is characterized by airway and pulmonary vascular dysfunction. Our initial studies in the baboon BPD model indicate that lung NO production and lung nNOS and eNOS expression are markedly attenuated during the genesis of BPD, and that NO replacement by inhalation (iNO) results in a sustained improvement in oxygenation index. The overall objective of this proposal is to investigate the role of alterations in NOS expression in the pathophysiology of BPD in the baboon model. The primary hypothesis is that pulmonary nNOS and eNOS expression are downregulated during the development of BPD, leading to diminished NO production and abnormal airway and vascular structure and function. The secondary hypothesis is that iNO reverses these abnormalities. Aim 1 is to define the normal ontogeny and changes in NOS protein and mRNA expression in early BPD, using approaches including laser capture microdissection to evaluate NOS mRNA levels in specific cell types harvested from frozen sections. Aim 2 is to determine the role of each NOS isoform in airway and vascular function in studies of NOS antagonism in intact animals. Aim 3 is to reveal the changes in exhaled NO levels with fetal development and BPD, as well as the contribution of each NOS isoform to exhaled NO. Aim 4 is to determine the effects of iNO on airway and vascular function, on the pulmonary course of early BPD, and on lung histology. Aim 5 is to determine the mechanisms underlying constitutive nNOS and eNOS gene expression in cultured baboon airway epithelium, and the role of cytokines in their downregulation. The results obtained will increase our knowledge of the role of NO in normally successful postnatal pulmonary adaptation and in the pathophysiology of BPD, thereby possibly leading to novel therapies for this devastating disorder.